This invention relates to an adhesive for ceramic articles and to a method for the adhesion of such articles, and more particularly to an adhesive advantageously useful for mutual adhesion of ceramic articles of nonoxide type materials such as silicon nitride, silicon carbide, and sialon or for adhesion of ceramic articles of such nonoxide type materials to ceramic articles of other materials and to a method for the adhesion of such articles.
Ceramic articles of nonoxide type materials such as silicon carbide, silicon nitride, and sialon excel particularly in strength at elevated temperatures, resistance to thermal shocks, and resistance to chemicals. By reason of these outstanding properties, they have come to attract keen attention as new high-temperature refractories dissimilar to ceramic articles of metal oxide type materials. Efforts are being continued for the development of new uses for the ceramic articles in various fields. For example, studies are being made to explore the feasibility of these ceramic materials in applications to high-temperature machines and implements, high-precision machine parts, heat exchangers, etc., to ceramic parts destined to be heated instantaneously to high temperatures, and to special high-temperature insulators.
For the ceramic articles of such nonoxide type materials to be advantageously utilized and enabled to manifest their characteristic properties to a full extent while in use, it is often necessary that their mutual adhesion or their adhesion to some other materials should be made while various machines and instruments and their parts which use such ceramic particles are in the course of fabrication. Development of a technique which provides ready and perfect adhesion even for ceramic articles of nonoxide type material finished in various shapes, therefore, is an indispensable requirement.
The ceramic articles of nonoxide type materials, however, have very poor affinity or wetting property generally for molten materials and, unlike the ceramic articles of metal oxide type materials such as alumina and magnesia, show a strong covalent bond property, offer very low reactivity with other chemicals and, moreover, possess a very small thermal expansion coefficient which is less than half the thermal expansion coefficient of alumina. Owing to these unfavorable attributes, adhesion of such articles is extremely difficult. In fact, the adhesion of ceramic articles of nonoxide type materials used to be performed solely by the hot press process involving application of very high levels of temperature and pressure until the methods described afterwards were introduced to the art. Since the hot press process must be operated under high temperature and high pressure, it can provide effective adhesion for ceramic articles of large sizes and complicated shapes only with extreme difficulty. Even if their adhesion is obtained, the adhesive strength with which they are held together is not entirely sufficient.
In view of the state of affairs described above, the inventors conducted research with a view to providing a novel adhesive capable of joining large ceramic articles of complicated shape with ample strength easily under mild conditions without resorting to the hot press method and a method for adhesion by the use of the adhesive.
Recently, an invention was perfected by some of the inventors of the present invention. This has been granted patent under U.S. Pat. No. 4,163,074 and 4,269,641 and is covered by Japanese Patent Application SHO 56(1981)-131683 (U.S. Pat. Application Ser. No. 407352) and SHO 56(1981)-194867 (U.S. Pat. Application Ser. No. 415,476).
U.S. Pat. No. 4,163,074 discloses a method which provides mutual adhesion of nitride ceramic articles by the steps of interposing a mixture of copper sulfide with kaolin intimately between the nitride ceramic articles, heating the joined nitride ceramic articles in a nitrogen-containing atmosphere, then overcoating the thermally treated ceramic articles with silver carbonate, and again heating them in a nitrogen-containing atmosphere.
U.S. Pat. No. 4,269,641 discloses a method for adhesion of two ceramic articles or adhesion of a ceramic article to a copper article. The adhesive used in this method is a paste obtained by mixing powdered CuS, powdered SiO.sub.2, and powdered LaCrO.sub.3, and blending the resultant mixture with an organic tackifier.
The methods of these U.S. patents have the following shortcomings. The method of U.S. Pat. No. 4,163,074 requires that adhesion be carried out under a high temperature of 1400.degree. C.-1500.degree. C. and a high pressure of 15 kg/cm.sup.2 in a vacuum or under an argon atmosphere. On the other hand, U.S. Pat. No. 4,269,641 requires the additional step of applying metal Cu powder to the surface of a mixture of CuS powder, SiO.sub.2 powder and LaCrO.sub.3 powder.
Japanese Patent Application SHO 56(1981)-131683 (U.S. Pat. Application Ser. No. 407352) covers an invention relating to a method which provides mutual adhesion of ceramic articles of both of silicon nitride by use of calcium fluoride or a mixture of calcium fluoride with kaolin.
The invention of Japanese Pat. Application SHO 56 (1981)-194867 (corresponding to U.S. Pat. Application Ser. No. 415,476) concerns a method which uses calcium fluoride and/or sodium fluoride and kaolin for adhesion of ceramic articles.
The inventors have further continued their study on alkali metal fluorides and alkaline earth metal fluorides except for the aforementioned fluorides and, consequently, found the following facts:
(A) Alkali metal fluorides and alkaline earth metal fluorides other than the aforementioned sodium fluoride and calcium fluoride exhibit similar activities.
(B) When any of the alkali metal fluorides (inclusive of sodium fluoride) is contained as an active component, the temperature of treatment (adhesion) can be lowered in proportion as the content of the alkali metal fluoride increases. Although having this great advantage, the adhesive layer produced tends to sustain minute pinholes to some extent and, depending on the particular place at which the product of adhesion is used, it is not necessarily impossible that these pinholes will have adverse effects unexpectedly upon the product of adhesion.
(C) The occurrence of such pinholes as mentioned above can be efficiently precluded by incorporation of at least one member selected from yttria and alkaline earth metal compounds.
This invention is based on this new discovery mentioned. Thus, it provides an adhesive which is effective in providing fast adhesion between articles of all kinds of ceramics, particularly nonoxide type ceramics, and suitable for a wide range of applications, and a method for the manufacture of this adhesive.